Device for reducing the relative humidity of a flowing gas

Abstract: The invention describes a device for reducing the relative humidity of a flowing gas. The device comprises a container (4) in which a tube (12) is arranged. A flowing gas passes into the container through a inlet (8) and is then forced down through a cavity in the container (6), whereupon the gas emits heat to the tube and the walls (15) of the container. As a result, fluid condenses from the fluid content of the gas. The flowing gas then flows up through a channel (14) in the tube (12) and is then warmed by the heat previously emitted to the tube (12), whereupon the relative humidity of the flowing gas declines.

Description: The present invention relates to a device for reducing the relative humidity of a flowing gas which is warmer than an ambient atmosphere, preferably gas expired by a living creature, comprising an inlet for the flowing gas, at least one flow-through channel in which the flowing gas is dehumidified and an outlet for the flowing gas. Dehumidification of a humid flowing gas is important in many contexts. For example, in conjunction with measurement of the flow of flowing gases, many flow meters operate less effectively, and may even produce erroneous measurements, when the flowing gas is so humid that fluid accumulates in the flow meter by condensation of moisture. Other measurement instruments, such as various gas analyzers, are also sensitive to condensation inside the measurement instrument. Preventing condensation of moisture into fluid, something which would accordingly affect the measurement instrument, is especially important in the respirator treatment of patients, since the respirator's function is based on correct measurement instrument operation. Gas expired by the patient is generally saturated with water vapor and also has a relatively high temperature, about 36° C. Ambient temperature around the patient where the measurement equipment is located is usually around 18-22° C. So cooling of expired air is generally unavoidable, and condensation therefore forms. US-A-4,182,129 describes a heat exchanger comprising a hollow housing in which a hollow element is coaxially disposed. Gas can enter the heat exchanger at first port. The gas will first flow between the interior wall of the housing and the exterior wall of the element and then withing the hollow element. The gas can exit it at a second port. A flow path withing the heat exchanger is thus formed. The hollow element is thermally conductive and connected to temperature regulating means to keep the element at a specific temperature. One known device for dehumidifying air expired by a patient is described in the brochure "Star Exhalation Isolation System Operating Instructions, Siemens Version, Infrasonic Inc., Form No. 9910053, April 1988. The device is connected to a ventilator's expiratory outlet in order to warm expired gas when it passes through the device. When expired air is warmed, the relative humidity of the expired air declines, reducing the risk of condensation inside a measurement instrument. The reduced relative humidity is because warm air has a higher saturation vapor pressure. One disadvantage of the device is that it requires an external power source and some kind of element for heating expired air. Moreover, the device must be placed close to the measurement instrument, since the heated gas would otherwise have time to cool again before reaching the measurement instrument. Another version of a device which reduces the risk of condensation of water vapor in expired air is described in an article entitled "Expired Gas Cooling Device" by J. Attwood and L. Bartel, both from the U.S. The device here comprises a cooling device for condensing water vapor in expired air before it reaches the measurement instrument. The expired air is cooled in order to reduce the risk of condensation inside the measurement instrument, even if air entering the measurement instrument may still have a very high relative humidity and may even still be saturated. Since a gas with a low temperature cannot hold as much water vapor as a warmer gas, relative humidity as such is not actually reduced when expired air is cooled. However, there is no additional cooling of the expired air and the risk of condensation inside the measurement instrument is therefore reduced. The cooling device also requires some external source of power. In this instance, a fan is powered which blows air across a radiator system through which expired gas passes. In addition to the fact that the described devices stop working when the respective power source (electric power supply) fails, the devices are also clumsy and difficult to clean. The object of the invention is to set forth a device, according to the preamble, which dehumidifies a flowing gas, in particular air expired by a patient, without being subject to the problems described above. One such device is achieved in accordance with the invention in that an element with a through channel is arranged in a container, whereby a cavity around the element in the container and the through channel jointly form the flow-through channel, the inlet is connected to the cavity at one end of the container, the outlet is arranged at one end of the element at the same end of the container, and the other end of the element is at a specific distance from the inlet, whereby the flowing gas passes into the cavity via the inlet, flows along the outside of the element to the through channel and passes through same to the outlet. When e.g. expired air passes into the cavity in the container via the inlet, the gas passes the entire element. Since expired gas is warmer than ambient air, some of the former's heat is transferred to the element. Gas simultaneously cools on the container's inner walls, and moisture in the flowing gas condenses into fluid on same. The fluid accumulates in the lowest part of the container. Cooled gas passing into the through channel in the element picks up some of the heat released by the inflowing moist gas to the element. The relative humidity of expired air therefore declines, and the air can pass on to the measurement instrument without any risk of condensation. For maximum utilization of the thermal effects, it is advantageous if both the element and container have thin walls and are made of a heat-conducting material. Depending on the element's length, it could be an advantage if the material's thermal conductivity were not too great. When thermal conductivity is great, there is always a risk of excessively rapid temperature equalization along the element, and heating of the gas flowing out towards the outlet would then decline. A material which conducts heat but does not pass it on along the length of the element would be ideal. It is hereby advantageous if the element is devised with a surface-enlarging exterior, since this will improve heat-extraction from gas flowing in the cavity. One appropriate way to increase the surface area of the element is to provide its exterior with projections. In the corresponding way, condensation on the inner walls of the container can increase if the container is devised with a surface-enlarging exterior, since this would increase cooling of the container's walls. In particular, the exterior of the container can be provided with projections, preferably flanges, to form the surface-enlarging exterior. It is also an advantage if the cavity, element and through channel have a cylindrical shape. A cylindrical shape will enable flowing gas to swirl inside the container down towards the through channel without blocking or impeding gas flow. With a cylindrical shape of the element, it will be an advantage if the projection is devised as a screw-shaped flange facing the cavity. As noted above, this would improve the exchange of heat between the downflow of gas in the cavity and the upflow of gas in the through channel. It is also an advantage if the container has a tap hole and if a collection vessel is connected to collect the fluid accumulating in the condensation of moisture in the flowing gas through the tap hole. The tap hole shall preferably be devised so gas flow is not affected by the volume of the collection vessel. For example, a wick or the like could be provided to conduct fluid down into the collection vessel. The collection vessel is preferably detachably connected so it can be periodically emptied of water during its use. An improvement of the invention is achieved in accordance with the invention in that the element comprises at least one additional through channel. With an additional through channel in the element, whereby the diameter of the channels could preferably be reduced, the heat-conducting surface area could be increased, without any increase in the size of the device and without any impediment to gas flow. Alternately, the same effect can be achieved in that at least one additional element with at least one through channel is connected in the cavity in the container to constitute the flow-through channel. With a plurality of elements, particularly those with a plurality of through channels, heat conduction and, accordingly, warming of the flowing gas can be optimized. An improvement of the device is achieved in accordance with the invention in that the element is thermally insulated from the container. In this way, heat extracted from the gas by the element will not be conducted to the walls of the container at the point of outlet attachment. At the same time, cooling of the container in relation to the surroundings will be more effective, and this will lead, in turn, to increased condensation of moisture in inflowing gas. The device according to the invention will now be described in greater detail, referring to the figures in which FIG. 1 is a partially sectioned side view of the device, and FIG. 2 shows an overhead view of the device. In FIG. 1 is shown a device 2 for dehumidifying a flowing gas and reducing the risk of condensation in sensitive measurement instruments located downstream from the device 2. The device 2 comprises a container 4 with a cavity 6. The flowing gas passes to the cavity 6 via an inlet 8. From the container 4, the flowing gas then exits through an outlet 10. A tube 12, thermally insulated from the container's 4 walls, is arranged in the container 4. The tube 12 has a through channel 14 which connects the outlet 10 to the cavity 6. Here, the flowing gas flows into the cavity 6 via the inlet 8 and continues down through the cavity 6, in contact with both the tube 12 and the wall 15 of the container 4 the whole time. When the device 2 is primarily intended for use in conjunction with the ventilation of a patient, the flowing gas consists of warm, expired air which is saturated with moisture. The warm air warms the tube 12 during its passage down through the cavity 6 and loses some of its heat. Heat is also lost to the walls 15 of the container 4. So fluid accumulates in the cavity 6 when moisture in the air condenses. Condensation takes place especially on the walls 15 of the container 4. The condensed moisture runs down into the bottom of the container 4 in which it passes through a tap hole 16 into a collection vessel 18 which is detachably mounted on the container 4. When the somewhat cooled air reaches the bottom of the container 4, it flows up through the through channel 14 in the tube 12. When the gas flows up through the channel 14, it comes into contact with warmer surroundings and is therefore warmed a little, thereby reducing he relative humidity of the gas. When the gas emerges from the outlet 10, it therefore has a lower temperature and a lower relative humidity, and the risk of condensation inside the measurement instrument downstream from the device 2 is therefore reduced. As can be seen in FIG. 2, the inlet 8 is eccentrically attached to the container 4, thereby facilitating the flow of gas down through the cavity 6 around the tube 12. In order to improve the exchange of heat between the gas and the tube 12, the tube 12 is equipped with a screw-like flange 20 along its longitudinal direction. The screw-like flange 20 increases the heat-absorbing surface for the passing gas flow. But, the screw-like flange 20 does not impede the flowing gas. Quite the contrary, this design facilitates through flow and reduces turbulence and the mixing of gas in the cavity 6. The tube 12 should be suitably made from a material with moderate thermal conductivity. Heat in gas flowing in through the inlet is to be extracted by the tube 12 but not conducted axially along the tube 12, as this would reduce the warming effect when gas flows towards the outlet. In principle, the tube 12 can be devised in sections which are thermally insulated from each other to prevent axial conduction of heat. The container 4 is also supplied with external flanges 22 in order to increase the heat-emitting surface area of the container 4. This means that the walls 15 retain the ambient temperature even when large flows of gas pass through the inlet 8. Alternative designs of the device are also possible. For example, the screw-like flange 20 and the flanges 22 can be excluded completely with no impairment in the function of the device 2. The container 4, the tube 12 and the channel 14 need not necessarily have a cylindrical cross-section. The device 2 can also be equipped with a plurality of tubes connected to the outlet, a single tube with a plurality of channels or any combination thereof. The cavity 6 can also be devised so it induces gas passing down through the container 4 to move in a helical whorl. In this instance, the tube 12 can also be devised in a similar manner. This would result in a longer heat-exchange distance without any increase in the external dimensions of the container. The essential feature for the device's function according to the invention is to utilize a thermal gradient between the flowing gas and the surroundings in order to first reduce the temperature of the flowing gas, without the need for any external sources of power, in order to achieve condensation and then increase the temperature by utilizing the gas's own heat, thereby reducing the relative humidity of the flowing gas.

Claims: A device (2) for reducing the relative humidity of a flowing gas which is warmer than an ambient atmosphere, preferably gas expired by a living creature, comprising an element (12) with a through channel (14) arranged in a heat conductive container (4), whereby a cavity (6) around the element (12) in the container (4) and the through channel (14) jointly form a flow-through channel (6, 14), an inlet (8) for the flowing gas connected to the cavity (6) at one end of the container (4) and an outlet (10) for the flowing gas at one end of the element (12) at the same end of the container (4) and the other end of the element (12) at a specific distance from the inlet (8), whereby the flowing gas passes into the cavity (6) via the inlet (8), flows along the outside of the element (12) to the through channel (14) and passes through same to the outlet (10) characterized in that the element (12) is thermally insulated from the container (4) and adapted to take up heat from the gas as it flows from the inlet (8) towards the other end of the element (12) and to return at least some of that heat to the gas as it flows from the other end of the element (12) towards the outlet (10). A device according to claim 1, characterized in that the element (12) is thin-walled and made of a heat-conducting material. A device according to claim 1 or 2, characterized in that the container (4) is thin-walled and made of a heat-conducting material. A device according to any of the above claims, characterized in that the element (12) is devised with a surface-enlarging exterior. A device according to claim 4, characterized in that the surface-enlarging exterior comprises projections facing the cavity (6). A device according to any of the above claims, characterized in that the container (14) is devised with a surface-enlarging exterior. A device according to claim 6, characterized in that the container (4) comprises projections, preferably in the form of exterior flanges (22), to increase its area of contact with the surroundings. A device according to any of the above claims, characterized in that the cavity (6) has a cylindrical shape. A device according to any of the above claims, characterized in that the element (12) and the through channel (14) have a cylindrical shape. A device according to claim 9, in combination with claim 5, characterized in that the element (12) has a screw-like flange (20), facing the cavity (6), which forms the projection. A device according to any of the above claims, characterized in that the container (4) has a tap hole (16), and in that a collection vessel (18) is connected to collect fluid, formed in the condensation of moisture in the flowing gas, through the tap hole (16). A device according to any of the above claims, characterized in that the element comprises at least one additional through channel. A device according to any of the above claims, characterized in that at least one additional element with at least one through channel is connected in the cavity in the container to form the flow-through channel. A device according to any of the above claims, characterized in that the element is subdivided into segments which are thermally separated from each other.

Inventors: LINDEN DAN, SCHNUERER MAGNUS, LINDEN, DAN, SCHNUERER, MAGNUS, Lindén, Dan, Schnürer, Magnus
Applicants: SIEMENS ELEMA AB, SIEMENS-ELEMA AB
Application year: 1994
Main classification: B01D5
IPC codes: B01D5, A61M16, B01D53, A61B5, F28D7